1. Field of the Invention
The present invention relates to hydraulic devices for heavy construction equipment, and more particularly to a variable priority device for the swing motor of heavy construction equipment.
2. Description of the Prior Art
Where at least two actuators are operated by a single pump, a "priority" is established to control the actuators such that one of the actuators is supplied with a larger amount of oil than the other actuator. For example, in an excavator wherein actuators respectively adapted to actuate a boom and a bucket are supplied with operating oil from a single pump, the amount of oil required for a boom operation is considerably larger than the amount of oil required for a bucket operation. To this end, a priority is assigned to the boom by limiting the amount of oil supplied to the bucket and increasing the amount of oil supplied to the boom by the limited amount of oil. Such a relationship is also applied to a case wherein the arm actuator should have a priority over the swing actuator.
In order to provide such a priority function, a stroke limiter or an orifice has been used. Referring to FIG. 1, an example of the stroke limiter is illustrated. As shown in FIG. 1, the stroke limiter, which is denoted by the reference numeral 101, is provided at a bucket-side switching valve 103 to limit the spool stroke of the switching valve 103 within a desired range, thereby preventing a supply oil passage 105 defined in the switching valve 103 from being fully opened. An example of the orifice is illustrated in FIG. 2. The orifice denoted by the reference numeral 201 in FIG. 2 is disposed in a bucket-side parallel oil line 203 to limit the amount of oil supplied to a bucket-side switching valve 205.
In either case, namely, where the supply oil passage defined the bucket-side switching valve is prevented from being fully opened by the stroke limiter or where the bucket-side parallel oil line is limited on the oil amount passing therethrough by the orifice, the amount of oil supplied to the cylinder for the bucket is reduced, while the limited amount of oil is additionally supplied to the other actuator, namely, the cylinder for the boom. In such a manner, the boom has a priority over the bucket.
However, the above-mentioned conventional devices have a problem that the oil passage or line is always limited on the oil amount passing therethrough, irrespective of whether the actuator associated operates alone or in combination with the other actuator. In other words, although the boom desirably has the priority over the bucket by the function of the stroke limiter or orifice when both the bucket and the boom operate in a combined manner, the bucket-side oil line is also limited undesirably on the oil amount passing therethrough even when the bucket operates alone. In the latter case, the limitation on the amount passing through the bucket-side oil line results in various problems such as unnecessary loss of pressure, heat generation caused by the overload and a decrease in the operating speed of the bucket.